Marine electromagnetic (“EM”) survey technology has been commercially used for locating hydrocarbon-rich subterranean features for less than 15 years. EM survey techniques typically employ generation of primary time-varying electromagnetic fields using dipole antennas. The primary time-varying electromagnetic field extends downward into the subterranean environment where it induces secondary currents. The induced secondary currents, in turn, generate a secondary time-varying electromagnetic field that is sensed, at various locations distributed across a relatively large area, in order to detect non-uniformities in the secondary electromagnetic field resulting from non-uniform electrical resistance in various features within the subterranean environment. Hydrocarbons and hydrocarbon-saturated rocks and sediments have much higher resistivities than water and water-saturated rocks and sediments. High-resistance subterranean pooled hydrocarbons and hydrocarbon-saturated rocks and sediments result in a non-uniform distribution of secondary current paths and concentration of electrical field lines in conductive portions of the subterranean environment above the pooled hydrocarbons and hydrocarbon-saturated rocks and sediments. By taking multiple measurements across a wide area for each of many different dipole-antenna transmitter locations, digitally encoded data sets are generated and stored in data-storage systems, which are subsequently computationally processed in order to provide indications of the longitudinal and latitudinal positions and depths of potential hydrocarbon-rich subterranean features. In many cases, three-dimensional plots, maps, or images, of the subterranean environment are generated as a result of these data-processing operations. The maps and images produced from EM-survey data can be used alone or in combination with maps and images produced by other methods, including marine exploration geophysical methods, to locate subterranean hydrocarbon sources prior to undertaking the expense of marine-drilling operations to recover liquid hydrocarbon from subterranean sources.
Because EM surveys have traditionally been conducted near the surface of an open body of water, such as an ocean, sea, or lake, the EM-survey data is often impacted by conditions at the water surface. For instance, swell noise can be a significant problem in offshore EM surveys. Swell noise results from swells, which are a series of surface waves that are not generated by a local wind and are often created by storms located hundreds or thousands of nautical miles away from the beach where they break. Because swells have dispersed from their source, swells typically have a longer wavelength than wind generated waves and have a narrower range of frequencies and directions than wind generated waves. As a result, swell noise is a high amplitude noise that can affect a number of neighboring traces and is often observed in geophysical images (e.g., seismic images) as vertical stripes or “blobs.” Those working in the petroleum industry continue to seek computational systems and methods that reduce swell noise in geophysical data used to create geophysical images created from EM surveys.